Coil solar receiver for a stirling disk and method for manufacturing  same

ABSTRACT

Coil solar receiver for a Stirling disk, the main components of which are pipes (where the sun hits), collectors, tanks and cupolas (of which there are two types, one that houses the regenerator, and the expansion cupola that is the area where the working gas is at a higher temperature) that includes a series of pipes ( 1 ) that perpendicularly exit from one collector ( 2 ) and perpendicularly enter another ( 2 ) and where each pipe consists of two semicircumferential curved parts ( 10 ) and three straight parts ( 11 ) parallel to one another: a center straight part (between curve and curve) and two straight parts on the ends (between the curve and the exit of one collector ( 2 )), the two straight end parts being located on the same plane, while the center straight part is located in a separate plane.

TECHNICAL FIELD OF THE INVENTION

The invention is framed within the technology of solar collectors and more specifically it is focused on the design of solar receivers for Stirling disk.

BACKGROUND OF THE INVENTION

The Stirling disk systems are electricity generation units that use solar radiation as a source of energy. The capacity of a single unit is between 3 and 50 kWe.

The Stirling disk systems transform with high efficiency the concentrated solar radiation into electrical energy. The essential components of the system are:

-   -   Parabolic solar concentrator.     -   Tracking system.     -   Stirling Engine with electric generator.     -   Solar heat exchanger (solar receiver).

The mode of operation of a Stirling disk system is the following: the concentrator reflects the solar radiation to the receiver which is located at the focal point of the concentrator. The solar radiation is absorbed in the receiver and this heats the gas (helium or hydrogen) of the Stirling engine at temperatures that are around 650° C. This heat is converted into mechanical energy in the Stirling engine. An electric generator converts this mechanical energy into electricity. So that the reflected radiation hits the focal point during the entire day, a solar tracking system continually moves the concentrator to follow the path of the sun.

The technology of the solar receivers is developed depending on the type of process in which it will be used, i.e., the type of plant and the cycle used. The invention filed relates to the solar receiver plant with disc and the cycle is Stirling. Of course, it is interesting to know the background and previous developments used in solar applications. The technologies used for solar plants in tower receivers represent a reference application.

In particular, two types of receiver systems are used for the Stirling parabolic disk:

-   -   External receiver systems.     -   Cavity receiver systems.

The external receivers have absorption surfaces in direct view with the concentrators and depend on the direct absorption of the radiation. The cavity receivers have, in turn, an opening through which the concentrated radiation passes to reach the surface of the receiver. The cavity ensures that most of the radiation that enters is absorbed by the internal surface of the receiver.

The receivers most widely used for the Stirling disk systems are the cavity receivers.

The receiver is located behind the opening to reduce the amount of lost heat and to decrease the intensity of the flow concentrated on its surface. The concentrated radiation that enters through the opening of the receiver is spread inside the cavity. Most of the energy is absorbed directly by the receiver, and virtually all the remaining energy is reflected or irradiated again within the cavity to be absorbed later.

In a cavity receiver, two methods for transferring the solar radiation absorbed to the Stirling engine working fluid have been identified.

The first method consists of using a receiver of directly illuminated pipes, where small pipes through which the work fluid of the engine circulates are placed directly in the region where the concentrated solar flow hits. The pipes form the surface of the receiver. In this way the working gas is heated as it passes through the interior of the pipes heated by the solar radiation.

The second method, the reflux method, uses a liquid metal as intermediate heat transfer fluid. The liquid metal is vaporized on the back surface of the receiver and is condensed in the pipes through which the working fluid of the engine circulates. That is, it absorbs the heat from the material that forms the receiver (which is hot by the exposure to solar radiation) and then transfers it to the pipes through which the working gas of the engine circulates. This second type of receiver is called reflux receiver because the steam is condensed and returns to be evaporated again.

An important factor in the design of the receiver is the exposure to severe conditions of operation together with cyclical conditions. The high temperature is the most important factor which, together with cycle operation, gives rise to the thermal fatigue of the components. Thermal fatigue is caused by the temperature cycles, from room temperature to operation temperature, both in the starts and stops and during the moments of cloudiness. This type of cycle can cause premature failure of the receiver. Within the receiver-cavity system, the receiver component is particularly sensitive. The design of receiver pipes, incorporating thin walls and operating at even temperatures during the transient, usually has fewer problems with thermal fatigue. The long term creep of the material of the receiver and the oxidation are important considerations for choosing the materials. Chrome and nickel super alloys (Inconel®), stainless steels, titanium and nickel alloys, nickel and cobalt alloys, etc are commonly used.

There are numerous documents which develop different receivers or aspects of the same in the state of the art. Some of them are pointed out in the following:

-   -   DE4433203 1996 HTC Solar Solar heater head for generation of         electric current from solar energy. Absorber with highly         conductive material and blackened (oxidized) to homogenize the         heat concentration. Preferably copper material, which is welded         to the pipes of the heat exchanger with high performance         stainless steel (as a sleeve of the pipe).     -   US2002059798 2002 Midwest Research Institute Disk/stirling         hybrid-receiver. Receiver hybrid system with sodium “heat pipe”.         Structure of the element made of nickel powder. It describes         integration with burner system (hybrid).     -   U.S. Pat. No. 6,735,946 2004 Boeing Direct illumination free         piston stirling engine solar cavity. Directly illuminated         piston. Without pipe exchanger. Arrangement of small pistons         concentrically to the solar beam. The receiver is a metallic         element made of highly conductive material such as copper,         nickel or graphite. An alternative to the design is to use “heat         pipes”.     -   U.S. Pat. No. 6,739,136 2003 Boulder, Colo. Arvada, Colo.         Combustion system for hybrid solar fossil fuel receiver.         Combustion system for hybrid solar fossil fuel receiver         comprising a premixer that combines air and fuel for forming the         mixture to be burned. There is a heat exchanger associated to         and in contact with the combustion chamber. This heat exchanger         provides the heat for the hybrid receiver when the heat from the         sun cannot be used as energy source.     -   U.S. Pat. No. 6,818,818 2004 Plano, Tex. Concentrating solar         energy receiver. It describes a system consisting of a high         reflectivity parabolic concentrator for reflecting the rays of         the sun on its concave side and a conversion module that         receives the concentrated solar radiation. In this conversion         module, there are two different receivers; a photovoltaic         receiver and a reception surface attached to a heat engine to         produce electricity.     -   EP0996821 2000 STM Corporation Heat engine heater assembly. An         equipment designed to use both solar radiation and the heat         produced by the combustion of natural gas as a power source for         a heat engine is described. A housing forming the receiver         allows the input of solar radiation until reaching the absorber.         Series of pipes external and internal to the receiver chamber         absorb the solar radiation and transmit heat to the fluid         flowing inside of it. A burner inside the chamber produces         combustion gases that also heat up these pipes.     -   U.S. Pat. No. 4,665,700 1987 STM United Stirling AB Hot gas         engine heater head. The object of the invention is to provide a         heating head in which the regenerators are connected to the         cylinders by pipes that surround these cylinders.     -   U.S. Pat. No. 4,602,614 1986 United Stirling, Inc. Hybrid         solar/combustion powered receiver. It is an improved receiver         which includes a heat exchanger inside the cavity with the pipes         tangentially spaced. There are multiple burners to provide a         path of the combustion gases and a window to seal the opening         and so there are no gas leaks out of the receiver.     -   U.S. Pat. No. 6,668,555 2003 Boeing Company Solar receiver-based         power generation system. This invention provides a design of         improved solar receiver that reduces the cost of such         mechanisms. The solar receiver includes a heat pipe that has a         fluid inside of it. The heat pipe has two condenser parts         arranged in two ends. Moreover, it includes an evaporator         between both ends. An air collector is attached to one of the         ends. This collector has an input and an output of air. A liquid         collector is attached to the other end, with its respective         input and output.     -   U.S. Pat. No. 4,911,144 1990 Stirling Thermal Engine, Inc         Spherical solar energy collector.

Invention relating to a collector for solar energy and in particular to one comprising an evaporator of a heat pipe-type heat transfer system

-   -   U.S. Pat. No. 4,475,538 1984 United Stirling AB Window for solar         receiver for a solar-powered hot gas engine. Solar receiver         which includes a window for the entry of the solar radiation as         an improvement.     -   CA2490207A1 2004 Shecs Labs—Solar Hydrogen Solar energy         collector. The invention is a receiver with an internal cavity         that reflects radiation. The receiver is contained in an inert         or reducing atmosphere to maintain the properties of the         reflective surfaces of the cavity. The heat absorption occurs in         pipes arranged symmetrically with respect to the main axis of         the receiver. In addition there is a quartz window in the input         of the device to reduce losses by convection.     -   DE19527272: Solarer Erhitzer für Stirling-Motoren. Solar heater         (1) for Stirling engine with a field of absorption (2) of         parallel pipes (8), which are connected to two collectors (4, 5)         through which the working gas circulates. The pipes (8) are         identical to each other and have loop-shape geometry.

In view of the existing state of the art, the present invention aims to provide a solar receiver that, overcoming the deficiencies found in the previous designs:

-   -   increases the resistance to thermal fatigue,     -   minimizes the shadows between pipes,     -   has directly illuminated pipes, to simplify the system avoiding         the inclusion of an intermediate heat transfer fluid and an         additional heat exchanger as well as to make the optical design         of the concentrator more flexible and that the engine can         operate in other positions different than with the sun behind         it,     -   does not leave gaps between the pipes when deforming by         expansions, escaping the concentrated solar radiation through         said gaps,     -   is easily weldable,     -   reduces head losses.

Thus, the new design allows to enhance the efficiency of the disc and to reduce the manufacturing and operation and maintenance costs. It also offers the possibility of:

-   -   easily refrigerating in the case of overheating (fan),     -   making the receiver independent from the cavity and the housing         of the insulation that could cover it, to make it more versatile         and facilitate the maintenance,     -   having the possibility to integrate a hybridization burner, i.e.         adding a gas burner to the Stirling engine to achieve a hybrid         plant where the burner must generate a flame that heats by         radiation and convection the pipes of the receiver; taking into         account the design of the coil receiver, the flame would be         housed in the gap defined by the pipes and heat transmission is         optimized,     -   simplifying the manufacturing processes and facilitating the         construction.

DESCRIPTION OF THE INVENTION

The invention consists of a new receiver for Stirling disk that complies with the requirements defined previously.

The designed receivers comprise the following components: pipes, collectors, tanks and cupolas.

-   -   Pipes: The pipes form the surface where the concentrated solar         light beam hits. They are arranged very close to each other so         that the surface has few gaps and the striking radiation can be         maximized.     -   Collectors: The collectors are the areas where the pipes are         welded such that the flow that runs through the pipes is         communicated with the tanks.     -   Tanks: The tanks are the internal areas of the collectors; from         here the working gas is distributed to each of the pipes.     -   Cupolas: There are two cupolas: the one for housing the         regenerator (or heat exchanger capable of absorbing heat from         the working gas, storing it and transferring it to the same to         close the cycle and it is, of the exchangers that the engine         contains, the one that handles a greater volume of thermal         power) and the expansion cupola (or dome cupola), which is the         area where the working gas is at a higher temperature.

The receiver design claimed in this invention, called coil receiver, is composed of a series of pipes that perpendicularly exit from one collector and perpendicularly enter another.

The surfaces where the pipes are welded to the collectors are parallel to one another and perpendicular to the entrance of the pipes.

Each pipe has two 180° curves along its length so that each pipe covers almost three times the distance between the two collectors.

Specifically, each pipe emerges from the origin collector perpendicular and in a straight line towards the target collector and before reaching the target collector turns 180° going slightly down in height and comes back in a straight line, by a horizontal plane parallel and below the departure one, towards the origin collector and before reaching the origin collector turns again 180° going down a little more in height, heading again in a straight line by a horizontal plane parallel and below the target collector, where it enters perpendicular and is connected by welding. In this way, the welding point of the pipe to the origin collector is higher in height than the welding point of the pipe to the target collector.

Therefore, each pipe consists of two semicircumferences (180° curves) and three straight parts which are: the central part, between curve and curve, and the two straight parts of the ends of the pipe, which are those that are connected to the collectors. The three straight parts are parallel to each other according to horizontal planes, since all of them are at different heights, while the straight parts of the ends are in the same vertical plane, parallel to the vertical plane containing the center straight part.

Observed from the direction in which the solar radiation hits, each pipe is arranged in such a way that the straight parts of the pipe form a surface without gaps between pipes and without shading of some parts over others.

There are two vertical rows of holes in each collector for welding all the pipes. In a row all the pipes that have that collector as origin are welded and in the parallel row all the pipes that have that collector as target are welded, so that they are interspersed. This arrangement prevents the pipes of one and the other row from coming into contact.

The design of the coil-shaped receiver allows that since the pipe is developed in two different planes, the expansions (due to high temperatures) are very slightly restricted, which initially will diminish the occurrence of stresses on the pipe. Therefore, a priori, this design will have few problems of breakage due to excessive stresses on the material.

The arrangement at various planes of the pipes allows better cooling of the receiver by means of a fan when the temperature of the material is excessively high. Since there is greater gap between pipe and pipe, the heat transfer by convection between the air from the fan and the outer surface of the pipes will be more effective. Since there is no area of the pipes shaded by other pipes the distribution of temperatures along the pipes is more uniform.

The surface viewed by the concentrated solar radiation beam is completely compact, i.e., there are no gaps between the pipes if viewed from the predominant direction in which radiation hits.

A drawback which can be found in this design is that when the length of the pipes increases and the area of global passage decreases (sum of the passage areas of all the pipes) the associated load loss increases. This could result in a not quantified loss of engine power. But the power loss mentioned above is outweighed by the increase in the temperature of the gas. Since it has pipes of greater length, the temperature reached by the gas after it passes through the receiver is higher for the same surface temperature.

Another further advantage is that the manufacture of the pipes does not involve technical complications. The simplest procedure would be to cut a straight pipe to the suitable length and then make the corresponding curves and angles to give it the desired final shape.

Also, the welding of the pipes to the collectors does not pose problems since the distance between the holes of the collectors is sufficient to manoeuvre in welding processes without problems.

Therefore and in view of the simulations performed, it can be concluded that the overall performance of this receiver model is much better than those known in the state of the art, since it efficiently distributes the stresses and deformations caused by the temperature since the pipes have more flexibility than what is known so far.

Another aspect that should be noted is that the contribution of the gas pressure to the stresses that the receiver must withstand is negligible with respect to the stresses caused by the high temperatures. It can be said that the designed models perform better than the previous ones against the thermal loads. However, there are critical points to fatigue in the area of connection of the pipes to the collector since a flexible area (the pipe) meets a much more rigid area (the collector). This poses a problem, since when the pipe expands it is constrained by the rigidity of the collector and a point where the fracture due to fatigue may appear is created.

There are several solutions to this localized problem:

-   -   To reduce the thickness of the area of connection of the pipes         to increase the flexibility of the collector in that area.     -   To place a sleeve at the critical point that serves as a         transition between the two areas of different rigidity.

In view of the results obtained for each of the three solutions adopted to improve the fatigue life it can be concluded that the best solution a priori is the placement of the sleeve since the number of cycles that it withstands is increased by a factor of five with respect to the case without sleeve. As for the manufacturing process of the receiver, it comprises the following steps.

-   -   Receiving the pipes     -   Bending of pipes     -   Assembly of pipes to collector with furnace welding     -   TIG Welding/plasma/laser for welding the tanks (made as junction         of plates) to the collectors     -   TIG Welding/plasma/laser for welding the cupolas (made from a         block of material) to the tanks.

Therefore, the receiver model shown as well as its manufacturing process, comply with the design constraints and improve the overall thermo-mechanical behaviour of the receivers known of the state of the art.

DESCRIPTION OF THE DRAWINGS

To complete the description that is being made and in order to help to a better understanding of the invention, a set of drawings is accompanied where with illustrative character and without limitation, the following has been represented:

FIG. 1: Solar receiver from the state of the art. Patent DE19527272.

FIG. 2: Plan, elevation and cross-section of the receiver of the invention

FIG. 3: Exploded view of the receiver of the invention

FIG. 4: Perspective view of a pipe

FIG. 5: Side view of a pipe

FIG. 6: Collector

The references used in the figures correspond to the following elements:

-   1. Pipes -   2. Collectors -   3. Expansion or dome cupola -   4. Cupola that houses the regenerator -   5. Plate for welding the collector -   10. Curved parts of each pipe -   11. Center straight part of each pipe -   12. Straight parts of the ends of each pipe -   20. Row of drill holes of the collector as origin -   21. Row of drill holes of the collector as target -   22. Drill holes of the collector

PREFERRED EMBODIMENT OF THE INVENTION

To achieve a better understanding of the invention, the new receiver designs for Stirling disk will be described next according to a preferred embodiment.

FIG. 1 shows the design of a receiver from the state of the art, corresponding to patent DE19527272, in which it can be seen that it is formed by the pipes (1), the collectors (2), the expansion or dome cupola (3) and the cupola which houses the regenerator (4). In this design the pipes (1) are identical to each other and have a loop-shaped geometry (note: the references that appear here do not correspond with those of the original document, they have been modified to achieve greater consistency with the description of the invention claimed in this document).

The preferred design of the claimed receiver and shown in FIGS. 2-6, complies with the following features:

-   -   Commercial materials used: Inconel® 625 or Multimet®.     -   Pipes to be used: outside diameter 3 mm. and thickness 0.5 mm.     -   Collectors on 3 mm plate.

FIG. 2 shows the different views of the coil model solar receiver and in FIG. 3 the exploded view. It is observed, that in the same way as in the patent of the state of the art of FIG. 1, the receiver consists of a series of pipes (1), which are attached at both ends to two collectors and resting on the regeneration (4) and expansion (3) cupolas. The biggest difference with the state of the art and where the breakthrough with regard to what exists is, lies in the design of the pipes (1). In the preferred embodiment, the receiver comprises 28 pipes (1) of a length of approximately 800 mm identical to each other and that perpendicularly exit from one collector (2) and perpendicularly entering another (2).

The surfaces or plates (5) where the collectors (2) are welded are parallel to one another and perpendicular to the entrance of the pipes (1).

The plan view of the invention (FIG. 2 below) as well as FIGS. 4 and 5 show the geometry of the pipes.

Each pipe (1) emerges from the origin collector (2) in a straight line (12) towards the target collector (2) and before reaching the target collector turns 180° (10) going slightly down in height and comes back in a straight line (11), by a horizontal plane parallel and below the departure one (12), towards the origin collector (2) and before reaching the origin collector it turns again according to a 180° curve (10) going down a little more in height, heading again in a straight line (12) by a horizontal plane parallel and below the target collector (2), where it enters perpendicular and is connected by welding.

Thus, each pipe has two 180° curves (10) along its length so that each pipe (1) covers almost three times the distance between the two collectors (2).

With this distribution it may be said that each pipe consists of three straight parts (12, 11, 12) and two curved parts (10).

In this preferred embodiment, the two curves (10) are semicircumferences with a radius of 11.1125 mm. and the straight parts are: the center (11) (between curve (10) and curve (10)) of 220 mm and the ends (12) (between curve (10) and exit or entrance of a collector (2)) of 255 mm (this includes the 3 mm of pipe (1) introduced in the collector (2) for welding).

The central part (11), between curve (10) and curve (10), and the two straight parts of the ends (12) of the pipe, are parallel to each other according to horizontal planes, since all of them are at different heights, while the straight parts of the ends (12) are in the same vertical plane, parallel to the vertical plane containing the center straight part (11).

Observed from the direction in which the solar radiation hits, each pipe (1) is arranged in such a way that the straight parts (11, 12) of the pipe form a surface without gaps between pipes and without shading of some parts over others.

FIG. 6 shows the collector (2). There are two vertical rows (20, 21) of drill holes (22) (each one with 14 drill holes) in each collector (2) where all the pipes (1) are welded. In a row (20) are welded all the pipes (1) that have that collector as origin and in the parallel row (21) are welded all the pipes that have that collector as target. This arrangement prevents the pipes of one (20) and the other row (21) from coming into contact.

The distance between centres of pipes (1) of the same vertical row is equal to six times the diameter of the pipe.

This system is specially designed for application in Stirling disk receivers but its extension to other fields of the industry that require similar features is not excluded. 

1. Coil solar receiver for a Stirling disk the main components of which are pipes, which form the surface where the concentrated solar light beam hits, collectors, which are welded to the pipes and place the fluid that runs through the pipes in communication with the tanks, tanks, which are the internal areas of the collectors from which the working gas is distributed to each of the pipes and cupolas, of which there are two types, the one that houses the regenerator and the expansion cupola that is the area where the working gas is at a higher temperature wherein it comprises a series of pipes wherein each pipe has a design such that it emerges from the origin collector in a straight line towards the target collector and before reaching the target collector turns 180° going slightly down in height and comes back in a straight line, by a horizontal plane parallel and below the departure one, towards the origin collector and before reaching the origin collector it turns again according to a 180° curve going down a little more in height, heading again in a straight line by a horizontal plane parallel and below the target collector, where it enters perpendicular and is connected by welding.
 2. Coil solar receiver for a Stirling disk according to claim 1 wherein the surfaces or plates where the collectors are welded are parallel to one another and perpendicular to the entrance of the pipes.
 3. Coil solar receiver for a Stirling disk according to claim 1 wherein each pipe is arranged in such a way that the straight parts of the pipe form a surface without gaps between pipes and without shading of some parts over others.
 4. Coil solar receiver for a Stirling disk according to claim 3 wherein it is formed by 28 pipes, each one of a length of approximately 800 mm.
 5. Coil solar receiver for a Stirling disk according to claim 4 wherein the two 180° curves are semicircumferences with a radius of 11.1125 mm. and the straight parts are: the center between curve and curve of 220 mm and the ends between the curve and the exit of one collector of 255 mm, including the 3 mm of pipe that are introduced in the collector for welding.
 6. Coil solar receiver for a Stirling disk according to claim 1 wherein the materials used are nickel-chromium-based alloys, as those marketed Inconel 625 or Multimet®.
 7. Coil solar receiver for a Stirling disk according to claim 1 wherein the pipes to be used have an outside diameter of 3 mm, and a thickness of 0.5 mm and the collectors are made of a 3 mm plate.
 8. Coil solar receiver for a Stirling disk according to claim 1 wherein there are two vertical rows of drill holes in each collector, in one row are welded all the pipes that have that collector as origin and in the parallel row are welded all the pipes that have that collector as target and the pipes of one and the other row do not come into contact.
 9. Coil solar receiver for a Stirling disk according to claim 8 wherein each vertical row of the collector consists of 14 holes.
 10. Coil solar receiver for a Stirling disk according to claim 8 wherein in each collector the distance between centres of pipes of the same vertical row is equal to six times the diameter of the pipe.
 11. Coil solar receiver for a Stirling disk according to claim 1 wherein a sleeve or pipe segment is placed, by welding, sintering or other methods, on each of the pipes of the receiver in the area of connection with the collectors, serving as a transition between the two areas with different rigidity.
 12. Method for manufacturing the coil solar receiver for a Stirling disk described in claim 1 comprising the following steps: Receiving the pipes Bending of pipes Assembly of pipes to collector with furnace welding TIG Welding/plasma/laser for welding the tanks, made as junction of plates to the collectors TIG Welding/plasma/laser for welding the cupolas, made from a block of material to the tanks. 